shards assignment to local Node when fanout flag is disabled

alerting/src/main/kotlin/org/opensearch/alerting/DocumentLevelMonitorRunner.kt

@@ -239,7 +239,19 @@ class DocumentLevelMonitorRunner : MonitorRunner() {
                     shards.remove("index")
                     shards.remove("shards_count")
 
-                    val nodeMap = getNodes(monitorCtx)
+                    /**
+                     * if fanout flag is disabled and force assign all shards to local node
+                     * thus effectively making the fan-out a single node operation.
+                     * This is done to avoid de-dupe Alerts generated by Aggregation Sigma Rules
+                     **/
+                    val localNode = monitorCtx.clusterService!!.localNode()
+                    val nodeMap: Map<String, DiscoveryNode> = if (docLevelMonitorInput?.fanoutEnabled == true) {
+                        getNodes(monitorCtx)
+                    } else {
+                        logger.info("Fan-out is disabled for chained findings monitor ${monitor.id}")
+                        mapOf(localNode.id to localNode)
+                    }
+
                     val nodeShardAssignments = distributeShards(
                         monitorCtx,
                         nodeMap.keys.toList(),

alerting/src/test/kotlin/org/opensearch/alerting/DocumentMonitorRunnerIT.kt

@@ -2750,6 +2750,68 @@ class DocumentMonitorRunnerIT : AlertingRestTestCase() {
         deleteDataStream(aliasName)
     }
 
+    fun `test document-level monitor fanout disabled approach when aliases contain indices with multiple shards`() {
+        val aliasName = "test-alias"
+        createIndexAlias(
+            aliasName,
+            """
+                "properties" : {
+                  "test_strict_date_time" : { "type" : "date", "format" : "strict_date_time" },
+                  "test_field" : { "type" : "keyword" },
+                  "number" : { "type" : "keyword" }
+                }
+            """.trimIndent(),
+            "\"index.number_of_shards\": 7"
+        )
+
+        val docQuery = DocLevelQuery(query = "test_field:\"us-west-2\"", fields = listOf(), name = "3")
+        val docLevelInput = DocLevelMonitorInput("description", listOf(aliasName), listOf(docQuery), false)
+
+        val action = randomAction(template = randomTemplateScript("Hello {{ctx.monitor.name}}"), destinationId = createDestination().id)
+        val monitor = createMonitor(
+            randomDocumentLevelMonitor(
+                inputs = listOf(docLevelInput),
+                triggers = listOf(randomDocumentLevelTrigger(condition = ALWAYS_RUN, actions = listOf(action))),
+                enabled = true,
+                schedule = IntervalSchedule(1, ChronoUnit.MINUTES)
+            )
+        )
+
+        val testTime = DateTimeFormatter.ISO_OFFSET_DATE_TIME.format(ZonedDateTime.now().truncatedTo(MILLIS))
+        val testDoc = """{
+            "@timestamp": "$testTime",
+            "message" : "This is an error from IAD region",
+            "test_strict_date_time" : "$testTime",
+            "test_field" : "us-west-2"
+        }"""
+        indexDoc(aliasName, "1", testDoc)
+        indexDoc(aliasName, "2", testDoc)
+        indexDoc(aliasName, "4", testDoc)
+        indexDoc(aliasName, "5", testDoc)
+        indexDoc(aliasName, "6", testDoc)
+        indexDoc(aliasName, "7", testDoc)
+        OpenSearchTestCase.waitUntil(
+            { searchFindings(monitor).size == 6 && searchAlertsWithFilter(monitor).size == 1 },
+            2,
+            TimeUnit.MINUTES
+        )
+
+        rolloverDatastream(aliasName)
+        indexDoc(aliasName, "11", testDoc)
+        indexDoc(aliasName, "12", testDoc)
+        indexDoc(aliasName, "14", testDoc)
+        indexDoc(aliasName, "15", testDoc)
+        indexDoc(aliasName, "16", testDoc)
+        indexDoc(aliasName, "17", testDoc)
+        OpenSearchTestCase.waitUntil(
+            { searchFindings(monitor).size == 6 && searchAlertsWithFilter(monitor).size == 1 },
+            2,
+            TimeUnit.MINUTES
+        )
+
+        deleteDataStream(aliasName)
+    }
+
     fun `test execute monitor generates alerts and findings with renewable locks`() {
         val testIndex = createTestIndex()
         val testTime = DateTimeFormatter.ISO_OFFSET_DATE_TIME.format(ZonedDateTime.now().truncatedTo(MILLIS))

sample-remote-monitor-plugin/src/main/java/org/opensearch/alerting/SampleRemoteMonitorRestHandler.java

@@ -136,7 +136,7 @@ public void onFailure(Exception e) {
             };
         } else if (runMonitorParam.equals("multiple")) {
             SampleRemoteMonitorInput2 input2 = new SampleRemoteMonitorInput2("hello",
-                    new DocLevelMonitorInput("test", List.of("test"), List.of(new DocLevelQuery("query", "query", List.of(), "test:1", List.of()))));
+                    new DocLevelMonitorInput("test", List.of("test"), List.of(new DocLevelQuery("query", "query", List.of(), "test:1", List.of())), true));
             BytesStreamOutput out1 = new BytesStreamOutput();
             input2.writeTo(out1);
             BytesReference input1Serialized1 = out1.bytes();
@@ -220,7 +220,7 @@ public void onFailure(Exception e) {
             sampleRemoteDocLevelMonitorInput.writeTo(out2);
             BytesReference sampleRemoteDocLevelMonitorInputSerialized = out2.bytes();
 
-            DocLevelMonitorInput docLevelMonitorInput = new DocLevelMonitorInput("description", index, emptyList());
+            DocLevelMonitorInput docLevelMonitorInput = new DocLevelMonitorInput("description", index, emptyList(), true);
             RemoteDocLevelMonitorInput remoteDocLevelMonitorInput = new RemoteDocLevelMonitorInput(sampleRemoteDocLevelMonitorInputSerialized, docLevelMonitorInput);
 
             Monitor remoteDocLevelMonitor = new Monitor(